vendingnick said:
about 15 years ago . I wrote to the paleontology department of the Natural history Museum in London . I said I was not happy with the idea of dinosaurs weighing 100 tons.
this was based on the fact that a structural surveyor had esimated the weight of my house at 100 tons. And a modern 50 ton battle tank uses a lot of fuel to get moving. essentially the fact that each leg of the 100 monster had to support 25 tons when perfectly balanced was a difficult to believe, I postulated that Dinosaurs could only have existed of this weight if the force exerted by gravity 250 million years ago was less than it is now.and the energy content and availability of vegetation much higher. my idea was ridiculed .in the reply I received . The museum stood by the fact of this heavy weights for dinosaurs. Soon after Fossils of even heavier 120 ton monsters were found. It now transpires that these weight estimations were wrong and over estimated, If a physicist had been left to test models of the limbs of these creatures for their load bearing and power requirements this fact could have been found out years ago.
There is a hypothesis that the gravitational field strength at the surface of the Earth has changed since early Mesozoic times, which is somehow related to the large sizes of reptiles on the earth. Although this theory is often discussed by fringe scholars, a few real scientists take it seriously.
This hypothesis has been discussed by such scientists as Pennycuick and Carey, as quoted in the references below. However, these authors are using scaling laws. Scaling laws are only reliable when the organisms involved a structurally similar despite their over all size.
There are deviations from the scaling laws when there are deviations in structure in any given group of animals. When comparing dinosaurs with mammals or tanks, one has to consider the special adaptations of dinosaurs. I don’t think there is any evidence that the size of animals is really determined by the strength of their bones. If this were so, then the bone structures of the largest animals would be fixed by the mechanical constraints. The bone structure of large animals is not fixed at all.
The structure of the bones in a group of animals can vary quite a lot even though their proportions follow very simple scaling laws. This suggests to me the opposite. There are factors other than the strength of the bones that limit animal size.
The dinosaur clade had a variety of bone structures. Many dinosaurs had hollow bones like birds. Some appear to have had solid bones like mammals. They also had a variety of gaits. In my opinion, this variation in structure and gait shows that gravity was not the limiting factor in dinosaur size.
Scaling laws are only applicable when the important limiting structures are the same in the group of animals. If gravity determined the size of dinosaurs, birds and mammals, then all dinosaurs, birds and mammals would have to have fundamentally the same bone structure.
Alexander (also see below) suggested that Tyrannosaurs may have moved faster than would be expected of an animal today. However, Tyrannosaur was an unusual dinosaur. They were built for power and speed in a way that other dinosaurs were not. Comparing all large dinosaurs to Tyrannosaurus would be like comparing a cheetah to a jaguar. Tyrannosaurs were likewise very different from other theropods.
Here are quotes from two scientists who believe that the gravitational strength of the Earth changed. As I said, I don't believe it. However, I find the mechanism of this change interesting.
These scientists think the density of the minerals in the mantle changed. I would love to know by what mechanism the mineral densities could change that much throughout the mantle. Maybe the KT asteroid had something to do with it?
“Newton Rules Biology” by C. J. Pennycuick (Oxfordd, 1992).
Page 6: There are also reasons to believe that the strength of gravity at the earth’s surface has varied by quite large amounts over geographical time, with effects that can be seen in certain fossil creatures.
Page 17: This rule also differs from the one for for walking and flying animals, whose stepping and flapping frequencies should vary with sqrt(g). …They are also also of interest in interpreting the mechanics of extinct animals, which lived at times when the earth’s surface gravity may have been stronger or weaker than it is now.
Page 18: Biologists are not the only ones who perceive the strength of gravity as a constant. Most geologists also believe that the strength of gravity at the earth’s surface has changed only minor amounts, if at all, over geological time. An exception is Carey (1976), who has assembled a mass of evidence that the earth’s radius is much larger now than it was in early Mesozoic times, probably due to a reduction in the density of the minerals making up the mantle. Carey cautions against too readily assuming that any of the earth’s physical characteristics, including its mass, radius, plane of rotation, and surface gravity….It would seem that gravity has not been constant throughout geoplogical time, although the inferred variations do not exactly coincide with Carey’s predictions.
“The Expanding Earth” by S.W. Carey (Elsiever, Amsterdam, 1976) .
http://palaeo.gly.bris.ac.uk/Palaeofiles/Tracks/Report7/speed.html
The fastest dinosaur that he recorded was moving at 3.6 ms-1. Alexander suggested this was more characteristic of the traditional lumbering image of dinosaurs.
Since the publication of this paper a number of other trackways have been examined. The equation developed by Alexander has undergone various corrections and modifications by Alexander and other authors (Thulborn, 1981; Farlow, 1981; Russell & Beland, 1976). Top speeds reported by authors have suggested some large theropods, such as Tyrannosaurus may have reached up to 10 to 15 ms^-1.
Alexander (1996) argued that based on the bone dimensions of Tyrannosaurus it is unlikely they could have traveled at more than 8ms^-1.
Please note that Alexander finds a problem only with Tyrannosaur. As I said, Tyrannosaur is an unusual dinosaur.
Alexander, R.M. (1976) Estimates of speeds of dinosaurs. Nature.Vol.261, pp.129-130.
Alexander, R.M. (1996) Tyrannosaurus on the run. Nature.Vol.379, pp.121.
Yes, many dinosaurs had hollow bones. The main reason it developed probably had more to do with respiratory limits than bone strength limits. However, hollow tubes carry more weight than solid tubes. High performance bicycles use double-butted frames, which in some ways are like the skeleton of some dinosaurs.
http://dinosaurs.about.com/od/typesofdinosaurs/a/Coelophysis-Facts.htm
“Granted, Coelophysis (pronounced see-low-FIE-sis) isn't very catchy, but the paleontologists of the mid-19th century adhered strictly to form when assigning names to their discoveries. The name Coelophysis was chosen by Edward Drinker Cope, who was referring to this dinosaur's hollow bones, an adaptation that helped it to remain light and nimble.”
Note that in this article, whales and dinosaurs are presented as satisfying the same power law. Whales have buoyancy to cancel gravity, while dinosaurs did not. Many dinosaurs had hollow bones, while whales do not. The scaling laws described here have nothing to do with the structural limits of bones.
http://www.nature.com/nmat/journal/v4/n6/box/nmat1408_bx1.html
“Finally, for a complete parallel of biology and mechanics, we expect a deviation from the 3/4 exponent towards 2/3 for small mammals (surface dominates at small scale) in the energy growth versus mass law. This deviation, nowadays still considered unjustified and unexpected, has been experimentally observed. It is clear that we would expect deviation towards the unitary exponent (where volume dominates) for very large mammals such as whales or dinosaurs.”
